Occlusive impedance phlebograph and method therefor

ABSTRACT

The venous patency of a human limb is assessed by measuring the venous outflow within a defined time interval after release of a forced blockage of the venous return to the heart and correlating it, where necessary, with the increased venous volume occasioned by the forced blockage. The volume changes and outflow rate are determined from electrical impedance measurements.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The invention relates to medical diagnostics and, more particularly, toassessment of the venous circulation of human limbs, particularly thelegs.

B. Prior Art

The human circulatory system is susceptible to numerous diseases anddisorders. Frequently, these disorders manifest a defect in thecirculatory system itself, while in other cases the disorder reflects adefect of primary origin elsewhere. In either case, it is desirable tobe able to rapidly and accurately assess the functioning of thecirculatory system of the patient.

The venous system of humans is particularly susceptible to seriouslydebilitating disorders arising from the gradual build-up of blood clotswithin the veins. In addition to obstructing the venous return to theheart, these clots sometimes reach a substantial size, break off, andthen travel to the heart and lungs where they block circulation andfrequently cause death. This complication can be prevented if clots aredetected and appropriate treatment is scheduled. Thus, it is highlydesirable to be able to determine whether one or more of these clots arepresent in the veins.

One technique I have heretofore utilized to assess venous patency is tomeasure the increase in venous volume accompanying a forced blockage ofthe venous return to the heart. This blockage may be applied by causingthe patient to inhale deeply, thereby increasing intraabdominal pressurewhich in turn restricts venous return, or by applying a force to thevenous system by means of devices such as a pressure cuff whichcollapses the veins and thereby blocks off the venous return. When thevenous return is blocked in this manner, the veins accumulate the bloodpumped into them from the arteries and store the excess blood byexpanding their cross section to thereby increase the stored volume. Theincrease in blood volume (hereinafter termed "venous capacitance") is ameasure of the distensibility of the veins and thus of the condition ofthe venous system.

Heretofore I have found that patients having a venous blockage such asis occasioned by a blood clot exhibit a noticably lower volume increaseaccompanying a forced blockage of the venous return than patients whoseveins are normal. In particular, I measured the change in impedancethrough a portion of the limb being tested in response to a forcedblockage of the venous return to the heart from that limb during deepbreathing and compared it to the impedance measured when the patient wasresting and absent any forced blockage, that is, the "resting baselineimpedance." I found that patients exhibiting an impedance change of lessthan 0.2% of the resting baseline impedance in response to a forcedblockage generally had one or more blood clots in their veins, whilepatients exhibiting an impedance change of greater than this amount weregenerally free of clots. This test has been extremely useful andbeneficial; however, in certain cases it produces incorrect results. Forexample, a patient who is hypovolemic (that is, having a low bloodvolume) exhibits a low volume change on blockage of the venous return,thereby indicating a blood clot in the veins when actually none ispresent. Additionally, the measurements obtained frequently variedsomewhat from observer to observer depending on judgemental factors ofthe observer in administering the test. Many sick or debilitatedpatients are unable to cooperate with the breathing maneuvers required.Thus, a test of greater accuracy and applicability is needed.

BRIEF DESCRIPTION OF THE INVENTION

A. Objects of the Invention

Accordingly, it is an object of the invention to provide an improvedmedical diagnostic method and apparatus.

Further, it is an object of the invention to provide an improved methodand apparatus for assessing the functioning of the human venous system.

Another object of the invention is to provide a method and apparatus forquickly and accurately assessing the functioning of the human venoussystem.

Still a further object of the invention is to provide a method andapparatus for quickly and accurately detecting the presence or absenceof a blood clot in the venous system.

B. Brief Summary of the Invention

In accordance with the present invention, I have found that the rate atwhich blood flows out from the venous system immediately followingcessation of forced blockage of the venous return to the heart(hereinafter termed "venous outflow rate") is indicative of thefunctioning of the venous circulatory system. When correlated with thechange in venous volume which accompanies the application of the forceblockage (the venous capacitance), the venous outflow rate provides anaccurate and reliable indication of the functioning of the venoussystem. Further, observation of changes in these measurements from onetime period to another allows early diagnosis of venous circulatorysystem problems.

Both the venous outflow rate and the venous capacitance are measured bymeasuring the electrical impedance through the limb being tested. Thisis accomplished in a known manner by applying a current through aportion of the limb positioned between a first pair of current-applyingelectrodes and measuring the voltage drop occuring in this limb portionbetween two voltage-measuring electrodes positioned intermediate thecurrent-applying electrodes. Typically, the voltage-measuring electrodesare positioned approximately 10 centimeters apart, and thecurrent-applying electrodes are positioned adjacent them. Preferably,the limb through which the impedance is measured is elevated above thehorizontal during these measurements so as to drain the pooled blood inthe veins. Also, the limb is advantageously slightly bent so as toprevent inadvertent compression of the veins in the limb which occurs insome patients when the limb is fulled extended (straightened).

To begin the measurement, the impedance through a portion of the limbwith the patient in a resting condition is established. This impedanceis known as the "resting baseline impedance," Z_(o) ; it provides ameasure of the blood volume within the limb during the resting (normal)condition. Typically it is of the order of 40 ohms. The impedancechanges hereafter described are expressed as a percentage of thisimpedance.

After the baseline impedance has been established the venous capcitanceis measured by blocking off the venous return to the heart and measuringthe resultant impedance change (in this case, a decrease) from theresting baseline impedance. The venous return may readily be blocked bycausing the patient to breathe in deeply, but it is more advantageouslyaccomplished by utilizing a pressure cuff placed around the limb andpositioned between the limb portion to be tested and the heart. Oninflation to a pressure greater than in the veins but less than in thearteries, the cuff collapses the veins and blocks off the venous return;the veins then accumulate (store) the blood pumped into them from thearteries in response to normal cardiac action.

In order to obtain accurate and repeatable measurements, the venousreturn should be blocked for a sufficient period of time to allow theveins to accumulate a substantial volume of excess blood in response tonormal cardiac reaction. I have found that a period of 45 seconds iswholly adequate for most purposes, although it may sometimes benecessary to extend this time to periods of up to a minute and a halffor patients with limited arterial input. Whatever the time intervalutilized, the change in impedance from the resting baseline impedanceover this interval is a direct measure of the venous capacity.

The venous outflow rate is next determined by releasing the venousblockage and measuring the impedance change over a suitably short timeinterval. Typically, over 80% of the excess blood volume in the veinswill drain from the veins within approximately 3 seconds from the timeof release of the blockage, absent a defect in the venous system; theremainder of the excess blood volume drains out at a slower rate. Invenous systems which are obstructed by a blood clot, however, asubstantially longer time interval is required to drain the venoussystem. Accordingly, the "short term" venous outflow rate (that is, theinitial outflow rate following release of the venous blockage)constitutes a significant parameter for differentiating normal venoussystems from abnormal venous systems.

The measurements required by the present invention may be performed in avariety of ways. Thus, they may be obtained from the tracings of astrip-chart recorder which continually traces the impedance through alimb section as the venous return from that limb is first blocked andthen unblocked. Preferably, however, the measurements are obtainedautomatically; this not only facilitates the utilization of the testingprocedure by unskilled and inexperienced personnel, but it also tends toinsure uniformity and repeatability of the measurements. In apparatus inaccordance with the present invention for automatically performing themeasurements, the venous blockage is performed by means of a pressurecuff placed around the limb to be tested. The cuff is controlled by atimer circuit which causes the cuff to inflate and remain so for apredetermined time interval; energizes a measuring circuit to establishthe impedance through the limb at the end of this time interval; causesthe cuff to deflate; and energizes a second measuring circuit todetermine the impedance change over a second time interval afterdeflation.

In one circuit performing these measurements, the difference betweenthese impedances (that is, the venous outflow rate) is determined andcompared with an index representative of the baseline impedance. As longas this difference is greater than this index, the circuit indicates theabsence of blockage or constriction of the veins. I have found that avenous outflow rate of less than about 0.3% Z_(o) per second (that is,about 0.12 ohms per second) nearly always indicates a venousconstriction or blockage such as from a blood clot, while a venousoutflow rate of greater than about 0.5% Z_(o) per second (that is, about0.20 ohms per second) nearly always indicates a healthy venous system.

Outflow rates within the range of from 0.3% Z_(o) per second to 0.5%Z_(o) per second are, by themselves, ambiguous. For venous outflow rateswithin this range, it is necessary to correlate the outflow rate withthe measured venous capacity. Thus, a small venous outflow rateaccompanied by a large venous capacitance is highly indicative of thepresence of a clot or a constriction in the veins, while a small venousoutflow rate accompanied by a small venous capcitance is quite likely tobe normal.

The results of the measurements described herein may be presented in avariety of ways. When a simple "normal/abnormal" decision is desired, anautomatic measuring circuit having a two-state output indicator such asa lamp, a buzzer, etc., may adequately display the measurement results.Alternatively, they may be plotted on a graph showing the venous outflowrate as a function of the venous capacitance; this allows rapidcomparison with measurements of other patients and also facilitatescomparison of subsequent measurements on the same patient. On such achart, it will be found that measurements of a large number of patientsnot only tend to segregate themselves in distinct areas on the chart,depending on whether the patient's venous circulatory system is healthyor not, but also that the measurements in each segment tend to clusteraround a regression line. Normally, this line has little significancefor a "static" test, that is, a single measurement of the venous outflowrate and venous capacitance. However, as will be described more fullybelow, it has substantial significance with respect to repeatedmeasurements over a time interval on the same patient.

The venous capacitance of patients often changes from day to day, wewell as from time to time during the day, in response to changes in bodymetabolism, physiology, etc. When the venous capacitance changes, thevenous outflow rate generally changes correspondingly. In patients withhealthy venous circulation, the changes in venous capacitance and venousoutflow rate generally bear a predetermined relationship to each other;experimentally, I have determined that these changes occur along aregression line defined by VOR = 0.59 (VC) (where VOR is the venousoutflow rate and VC is the venous capacitance) in healthy circulatorysystems, and along the line of much smaller circulatory systems whichare constricted or blocked by a clot. Thus, by repeating measurements ofvenous outflow rate and venous capacitance on a patient, one can obtainan early indication of adverse changes in the venous circulatory systembefore these changes become significant enough to result in ameasurement outside the normal range for a single measurement.

As an example of the results obtained with the present invention, in 113patients examined for possible blockage of the major veins of the legand proven not to have such blockage by other techniques (such as byvenous phlebography), the present invention clearly indicated the lackof an obstruction in 117, an accuracy of approximately 96%. Rechecks ofthe patients showing abnormal measurements resulted in normalmeasurements, as may sometimes occur when the original test conditionsinduced error, e.g., by improper leg positioning or other such factors.Conversely, in 49 patients having proven recent clots in the popliteal,femoral, or iliac veins, the method of the present invention correctlydiagnosed 48 as having a substantial blood clot, an accuracy ofapproximately 95%. in the 49th patient, the clot was in such a position(in the hypogastric vein and propagating into the common iliac vein) andof such a size (obstructing not more than 60% of the lumen) as not to bereadily detectable by impedance techniques. Thus, the method has beenproven accurate in detecting the presence of a fresh clot in the majorveins of the leg in tests on 161 out of 166 patients. It should be notedthe method of the present invention relies upon a noticable change inthe hemodynamics of the venous system for detecting abnormalities. Thus,minor malfuctions in the system, such as the presence of small blockagesof the calf veins, may not be detected. However, these disorders are notthemselves dangerous, but become dangerous only as they enlarge andpropagate into larger veins. As they enlarge these veins, they do indeedaffect the hemodynamics of the venous circulatory system and thus arelikely to be detected by the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing and other and further objects and features of theinvention will be more readily understood from the following detaileddescription of the invention when taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a pictorial view of a patient undergoing a test for venouspatency of the left leg;

FIG. 2 is a reproduction of a tracing of the impedance through the limbof the patient in response to an initial blockage of the venous returnto the heart and the subsequent release thereof;

FIG. 3 is a diagram illustrating the relation between the venous outflowrate and venous capcitance and indicating the ranges of "normal" and"abnormal" with respect to substantial blood clots in the venous system;

FIG. 4 is a circuit diagram accompanied by a timing diagram of oneapparatus for measuring the venous outflow rate alone; and

FIG. 5 is a circuit diagram, accompanied by a timing diagram, of anapparatus for measuring the venous outflow rate as well as venouscapacitance and comparing the two.

In FIG. 1 a patient 10 whose leg veins are to be tested for venouspatency is positioned on a mattress 12 supported by frame 14 whichelevates a leg 16 to be tested. Conveniently, the mattress 12 may beinclined at an angle of approximately 20° to the horizontal; elevatingthe mattress in this manner drains the pooled blood from the leg veinsof the patient being tested, and assists in providing a standardreference for measurement of the desired circulatory parameters, that isvenous capacitance and venous outflow rate. A pressure cuff 18 ispositioned around the thigh just above the knee; it is connected to asphygmomanometer 20 and is inflated by a squeeze bulb 22. Initially, thecuff is deflated.

A pair of electrode sets 24, 26 is positioned on the calf of the limb tobe tested, and is connected to a measuring or recording instrument 28.The leg 16 is positioned to remove external forces which might constrictthe veins or otherwise obstruct the venous return to the heart. For thispurpose, the leg is slightly bent and the hip of the patient rotatedslightly outwardly to relieve any longitudinal tension on the veins orany extraneous compression which might be applied to the veins by thebones of the knee joint. For purposes of illustration, the instrument 28is depicted as a strip chart recorder having a laterally movable pen 30which traces a record on a longitudinally movable chart 32. A typicalrecord formed by such a recorder will be described in detail inconnection with FIG. 2

The electrode pairs 24 and 26 each preferably consist of a pair ofelectrodes parallel to, and slightly spaced apart from each other. Theoutermost electrodes 24a and 26a drive a standardized high frequency,alternating current through the leg section between them, while theinnermost electrodes 24b and 26b measure the voltage drop through thelimb section in response to this current. The magnitude of the voltageand current define the impedance of the venous system of this legsection and this impedance is recorded by means of the recorder 28. Theimpedance is a function of the cross section of all the electricallyconductive paths in the leg, including the arteries and veins. Theconductivity of the arteries and veins changes with variations in bloodvolume, but the conductivity of all other electrically conductive pathsremains essentially constant. Because of the substantially larger crosssection of the veins as compared to the arteries, the changes inconductivity (and thus its reciprocal the impedance) due to blood volumechanges is determined largely by the veins.

To begin the test, the impedance through the leg is first establishedwith the patient in a resting condition; this is the resting baselineimpedance. This impedance is of considerable magnitude (typically,approximately 40 ohms) in comparison to the magnitude of the impedancechanges to be measured during the test (typically, tenths of ohms) andthus it is desirable to utilize an instrument establishing the restingbaseline impedance as the "zero level" from which impedance changes aremeasured. An instrument which is specially suited for this purpose isthe well known impedance bridge; when this bridge is set to balance atthe resting baseline impedance, only the deviations from this impedancelevel are provided as output. Accordingly, in the discussionhereinafter, it will be assumed that the instrument 28 incorporates animpedance bridge such that it registers on the strip chart 30 only thedeviations from the resting baseline impedance.

After the resting baseline impedance has been established, the venousreturn to the heart is blocked by inflating the pressure cuff 18 to anextent sufficient to collapse the veins and cut off the venous return.When this is done, the blood pumped into the veins of the leg fromnormal cardiac action accumulates in the veins, thereby increasing theircross section and decreasing the impedance measured through them. Thischange in impedance is shown clearly in FIG. 2 of the drawings in whichan upper tracing, 32a, shows a typical impedance through the legs inresponse to the initial application, and subsequent release, of a forcedblockage of the venous return in the leg of a patient having nomalfunction of the venous system, (hereafter referred to as "normal")while the lower tracing, 32b, shows a typical impedance tracing underthe same conditions in the leg of a patient having a venous clot(hereafter referred to as "abnormal"). In the drawings, a decrease inthe impedance is represented by an upward excursion of the tracing,while an increase in impedance is represented by a downward excursion ofthis tracing. The scale is such that one vertical division (5mm on therecorder chart) equals 0.2% Z_(o) where Z_(o) is the resting baselineimpedance which may vary from 20 to 90 ohms in extreme cases buttypically is of the order of 40 ohms. One horizontal division equals 1.5seconds.

The venous blockage is imposed at time T₀ and is maintained for anextended time until T₁. The time interval T₁ - T₀ should include asubstantial number of cardiac cycles so that a substantial quantity ofblood is pumped into the veins during this interval. In the tracingsshown in FIG. 2, the blockage was maintained for a period of 45 seconds.During this time, the impedance through the normal leg dropped from Z₀to Z₁, while the impedance through the leg containing a venous disorderdropped from Z₀ 'to Z₁ '. This corresponds to a volume increase of ΔV,for the normal leg and ΔV', for the abnormal leg. In the examples shownin FIG. 2, the venous capacitance of the normal leg is approximatelytwice that of the abnormal leg.

At T₁ the cuff is deflated. In response to this, the leg veins begin toempty their stored excess blood volume and the impedance correspondinglyincreases toward the resting baseline impedance from FIG. 2. It will benoted that the normal leg empties its excess blood volume at a muchfaster initial rate than the abnormal leg; in the example shown, theinitial outflow rate in the normal leg is approximately three and a halftimes the initial outflow rate in the abnormal leg. The initial outflowrate provides a significant measure to the patency of the venous system.This outflow rate is advantageously computed over a time interval of theorder of approximately 3 seconds, an interval which is short enough tomeasure an outflow rate which is quite close to the maximum outflow ratethroughout the interval, while long enough to provide a sufficientresolution to thereby insure accuracy and repeatability of themeasurements.

Turning now to FIG. 3, a plot of the venous outflow rate (verticalscale, calibrated in percent of the patient's resting baseline impedanceper three second interval) versus the venous capacitance (horizontalscale, calibrated in percent of the patient's resting baseline impedanceis shown). The closed dots represent measurements of impedance throughthe legs of patients in which the absence of any venous restrictions orclots was independently confirmed by other methods; the open dotsrepresent measurements of impedance through legs of patients in whichthe presence of a venous clot or restriction was independently confirmedby other methods.

It will be seen from FIG. 3 that the normal measurements (thoseindicating the absence of a venous disorder) group themselves into agenerally separable region of the graph from the abnormal measurements(those indicating a venous disorder, in this case, a blood clot). Forexample, one may demarcate the two groups by enclosing them withinseparate generally smooth contours, each encompassing all the pointswithin one group and as few as possible points from the other group. InFIG. 3, such contours are shown enclosing the abnormal region designated"A" and the normal region designated "B." This creates a zone ofambiguity, designated "C" in FIG. 3, which contains three abnormal andone normal measurement. Within these zones the normal measurementscluster along a regression line 40 given by VOR = (0.59 VC + .48)% per 3second interval, while the abnormal measurements cluster along a linegiven by VOR = (0.23 VC + 0.14) percent per 3 second interval.

Alternatively, it will be noted that a horizontal line at approximately1.2% Z_(o) per 3 second interval (that is, 0.4% Z_(o) per second, whereZ_(o) is the resting baseline impedance) distinguishes most normalmeasurements from abnormal measurements, and incorrectly categorizesonly a single measurement. An uncertainty zone of from 1.0% to 1.4% per3 second period may desirably be provided for in which all measurementsare confirmed by repetition. Thus, the venous outflow rate provides anindex of substantial sensitivity to help distinguish normal measurementsfrom abnormal measurements. However, the venous outflow rate may varyconsiderably from patient to patient, as well as from time to time in apatient depending on the patient's venous capacitance. Thus, comparisonof the venous outflow rate with the venous capacitance provides a morereliable indicator of the functioning of the venous system.

This is especially the case when measurements on a particular patientare repeated over a period of time and a history of the changes in thesemeasurements is followed. In these situations it will be found that thevenous outflow rate changes quite rapidly with changes in venouscapacitance in patients with incipient disorders of the circulatorysystem (e.g., as shown by the rectangles in FIG. 3 in a hypotheticalcase) and changes less rapidly with changes in venous capacitance innormal patients (e.g., as shown by the triangle in FIG. 3 in a furtherhypothetical case). Indeed, the latter changes typically occur alonglines parallel to the regression line 40 of FIG. 3. Thus, repeatedobservation on the same patient can provide early diagnosis of incipientcirculatory disorders which are not yet of a magnitude sufficient todepart from the normal regions of FIG. 3.

Turning now to FIG. 4, there is shown a circuit for automaticallymeasuring the venous outflow rate and venous capacitance over a definedtime interval and comparing them with a selected index. The circuitcomprises an impedance bridge 50 connected through gates 52 and 54 tosample and hold circuits 56 and 58. The bridge 50 is balanced to thepatient's resting baseline impedance at the start of the test. A typicalsample and hold circuit 56 comprises an amplifier 60 having theconventional inverting and non-inverting input terminals 60a and 60b,respectively; a resistor 62 connected to the inverting input terminal60b; a ground lead connected to the non-inverting input terminal 60a; acapacitor 64 connected between the inverting input terminal and theoutput terminal; and a switch 66 connected across the capacitor. Theamplifier 60 is a high gain, high input impedance, low output impedance("operational") amplifier and the time constant of the R-C combinationof resistor 62 and capacitor 64 is low to minimize sampling errors. Thesample and hold circuit 58 is similarly constructed and it will not bedescribed in detail.

The outputs of the circuits 56 and 58 are applied to the inverting andnon-inverting inputs, respectively, of an operational amplifier 70through resistors 72, 74, respectively. Feedback resistors 76, 78 areconnected between the output terminal of amplifier 70 and thecorresponding input terminals and determine the gain of the amplifierfrom the input terminals thereof to the output terminals in combinationwith the resistors 72 and 74. For reasons described below, this gain isset to a value of k, where k is the index (here, 1.2% Z_(o) per 3 secondinterval) separating normal from abnormal outflow rates. The output ofthe amplifier 70 is applied to the noninverting input of an operationalamplifier 80 which serves as a comparator; a reference voltagecorresponding to the impedance Z_(os) (for purposes of illustration,shown as a battery 82) is applied to the inverting input thereof. Thecomparator 80 drives the toggle input of a flip flop 82 which isnormally in the reset state. It is changed from this state only if thetoggle input is positive at the time it is strobed at its strobeterminal 82a. The Q output of the flip flop drives an indicator lamp 84;similarly, the R output drives a lamp 86. The circuit also includes atimer 90 which controls the measurements made by the circuit, and apressure cuff 92 corresponding to cuff 18 of FIG. 1 and includinginflation means (e.g., a source of compressed air and an electronicvalve connecting the air to the cuff when open and venting the cuff toatmosphere when closed) which is inflated in response to a signal fromtimer 90 and deflated thereafter.

Referring now to FIG. 5 in conjunction with FIG. 4, the test is begun bysetting bridge 50 to the resting baseline impedance of the patient whosevenous system is being tested. Preferably this is accomplishedautomatically by the bridge 50 in response to a control signal emittedby timer 90 and shown as signal "A" in FIG. 5. Thereafter, bridge 50tracks the impedance excursions from the patient's resting baselineimpedance. After this is accomplished, the timer 90 applies tocuff-inflation signal (signal "B" in FIG. 4A) to pressure cuff 92 tocause this cuff to inflate and remain in the inflated state for theduration of the signal; this blocks the venous return to the heart. Forthe reasons previously described, this signal should have a duration ofthe order of 45 seconds or more in order to insure a substantialdistention of the veins.

At the end of the inflation period, and immediately before the pressurecuff 92 is deflated, the timer 90 emits a measurement signal (designated"C" in FIG. 5) which opens gate 52 and connects the sample and holdcircuit 56 to the impedance bridge 50. The duration of the samplingsignal C should only be long enough to allow the sample and hold circuit56 to obtain an accurate indication of the impedance Z₁ measured by thebridge 50 at the time the sampling signal is generated. The sample andhold circuit 56 samples the instantaneous magnitude of the impedancemeasured by the bridge (which is actually the impedance change from thepatient's resting baseline impedance) and provides as output a voltageindicative of this magnitude. This voltage is held for subsequentprocessing.

At a defined time after the end of the cuff inflation signal B (FIG. 5)and thus the end of the first measurement, the timer 90 emits a secondmeasurement signal D (FIG. 5) which opens gate 54 and connects sampleand hold circuit 58 to impedance bridge 50. This event occurs at a timerT₂ which is preferably approximately 3 seconds after the deflation ofthe pressure cuff. The sample and hold circuit 58 registers theimpedance Z₂ measured by bridge 50 at the time T₂ and provides an outputvoltage indicative of this value. The scaled difference 1/K (Z₂ - Z₁) isformed by the amplifier 70 and applied to the comparator 80 where itsmagnitude is compared with the magnitude of the index established bybattery 82. If the impedance difference is greater than the index, thatis, if 1/k (Z₂ - Z₁) > Z_(os) and thus (Z₂ - Z₁) > k Z_(os) where k isapproximately 1.2, the output of amplifier 80 is highly positive and theflip-flop 82 lights lamp 84 when it is strobed by strobe signal Eemitted by timer 90. This indicates a normal venous system. Conversely,if the impedance difference is less than the index established bybattery 82, the output of the amplifier 80 is highly negative and flipflop remains in the reset state when strobed. Thus, lamp 86 lights toindicate an abnormal venous system. The system is reset in response totiming signal F(FIG. 5) which dumps the capacitors of the sample andhold circuits 56 and 58 by closing the shorting switches across them.

Numerous alternatives for carrying out the tests described herein willsuggest themselves to those of ordinary skill in the art. For example,the venous outflow rate and venous capacitance may be measured bytechniques other than electrical impedance techniques. For example, limbvolume changes may be measured pneumatically or hydraulically and thepneumatic or hydraulic analagy to FIG. 3 may be established to determinethe corresponding index for assessing venous patency. Similarly, afunction generator maybe employed to simulate the areas designated A andB in FIG. 3 for testing measured values of the various measurements ofvenous outflow and venous capacitance to determine whether themeasurements lie within the normal or the abnormal range. Othermodifications will suggest themselves to those or ordinary skill in theart in carrying out the method in the present invention.

SUMMARY

From the foregoing, it will be seen that I have described a method andapparatus for accurately and rapidly assessing the patency of the venoussystem and, in particular, that of the major veins (popliteal, femoral,and iliac) of the venous system in the lower limb. The method is simpleto utilize, quickly performed, and highly accurate. Apparatus is alsoprovided for performing the measurements and registering the results.

Having illustrated and described my invention, I claim:
 1. A method ofassessing the patency of the venous system in an animal body limb,comprising the steps ofA. blocking off the venous return to the heartfrom the limb to be tested, B. maintaining said blockage for a period oftime sufficient to accumulate excess blood in said limb portion, c.releasing said block age and measuring the rate of venous outflow fromsaid limb occurring within a defined time interval in response to saidrelease, and D. registering said system as healthy when said rate isgreater than the rate defined by (0.60 VC + 0.48), where VC is thevenous capacitance, and registering said limb as diseased when said rateis less than the rate defined by (0.20VC + 0.14).
 2. A method accordingto claim 1 in which said blockage is maintained for a period equal to asubstantial number of cardiac cycles.
 3. A method according to claim 2in which said blockage is maintained for a period in excess of 30seconds.
 4. A method according to claim 1 in which the defined timeinterval over which the outflow rate is measured is small compared tothe time during which the blockage is maintained.
 5. The method of claim4 in which the defined time interval is less than 5 seconds.
 6. Themethod of claim 5 in which the defined time interval is approximately 3seconds.
 7. The method of claim 1 which includes the step of drainingblood from the limb prior to imposing a blockage therein.
 8. The methodof claim 7 in which said drainage is accomplished by elevating the limbabove the horizontal.
 9. The method of claim 8 in which said limb isblocked by applying a pressure cuff to the limb at a positionintermediate the heart and that portion of the limb to be tested andthereafter inflating said cuff to thereby block off the venous return tothe heart.
 10. The method of claim 9 in which said limb comprises a legand in which the pressure cuff is placed immediately above the knee onthe leg.
 11. The method of claim 1 in which the rate of venous outflowis measured by measuring the rate of change of electrical impedancebetween at least two electrodes placed on the limb portion to be tested.12. The method of claim 11 in which the absence of a venous blood clotis to be determined and the range of rates to which the venous outflowrate is compared comprises those rates corresponding to electricalimpedance changes of approximately 0.4% Z_(o) per second.
 13. The methodof claim 11 in which the presence of a venous blood clot is to bedetermined and the range of rates to which the venous outflow rate iscompared comprises those rates corresponding to electrical impedancechanges of approximately 0.4% Z_(o) per second or less.
 14. The methodof claim 1 which includes the step of measuring the venous capacitancein response to the forced blockage and comparing it to the venousoutflow rate following release of said blockage, a measurement of venouscapacitance and venous outflow rate lying within the region labelled "A"in FIG. 3 indicating a venous blockage and a measurement of thequantities lying within region "B" of FIG. 3 indicating absence of sucha blockage.
 15. Apparatus for assessing the patency of the veins in ananimal body limb comprising:A. means for measuring the venous outflowfrom said limb during a given time interval following release of aforced blockage of the venous return to the heart and, B. meansresponsive to the measured outflow to provide an output indicative ofwhether or not said outflow is less than the rate defined by (0.20 VC +0.14) where VC is the venous capacitance.
 16. Apparatus according toclaim 15:1. including means for measuring an increase in venous volumefollowing said venous blockage; and
 2. in which said indicating means isresponsive to said increase measuring means to indicate whether both theincrease and decrease bear a defined relationship to each other.